1. Field of the Invention
The present invention relates to an optical information medium formed by bonding two optical information substrates with each other; a method for producing the same; and a unit for producing the same. The present invention also relates to an optical information medium having two information signal layers for recording/reproducing information thereon/therefrom by irradiating and focusing a laser beam thereon; a method for producing the same; and a unit for producing the same.
2. Description of the Related Art
Recently, various optical information media such as an optical card and an optical disk have been practically used. A compact disk (hereinafter, simply referred to as a "CD"), in particular, has become remarkably popular. The video and audio information recorded as digital signals on a CD-ROM can now be processed easily by a computer. As a result, the value and the utility of a CD have been considerably increased in the information industry.
In order to record/reproduce high-density information onto/from an optical disk, it is necessary to shorten the wavelength of the laser beam to be used for the recording and the reproduction and increase the numerical aperture (NA) of an objective lens. Thanks to the remarkable development in the technologies for producing semiconductor laser devices, video compression technologies and various kinds of peripheral technologies, it is currently possible to record video and audio information on one side of a disk with a diameter of 120 mm for a long time. For example, by using an objective lens with an NA of 0.6 and a red semiconductor laser with a wavelength of 650 nm, it is possible to reproduce information from the signal pits with a track pitch of 0.74 .mu.m and a pit length as short as 0.40 .mu.m in a highly reliable manner. If an 8-15 system is used as a modulation system, then one bit length becomes 0.25 .mu.m. Therefore, it is possible to accomplish a density higher than that of a CD by 5 times or more. By setting the redundancy to be about 15%, information having a capacity of about 5 Gigabytes can be recorded on a disk with a diameter of 120 mm, and video and audio signals in compliance with MPEG 2 standard can be recorded at 4.7 Mbps on average on one side of a disk with an equal size to that of a CD for about 142 minutes.
However, the allowable disk tilt of an objective lens with an increased NA is very small. For example, if a substrate with a thickness equal to that of a CD, i.e., 1.2 mm, is used, then the tilt allowable for an objective lens with an NA of 0.6 is about 0.25 degrees. The tilt is no larger than the error to be caused in installing an optical head onto a player. Therefore, if a tilt is caused by a deformation on the disk, then such a tilt is not allowable. Accordingly, it is difficult to practically use an objective lens with such a small tilt.
By thinning the thickness of the substrate, such an objective lens with an increased NA can be used and a practical high-density optical disk can be accomplished. For example, by reducing the thickness of the substrate to one half of the thickness of a CD, i.e., 0.6 mm, the tilt allowable for an objective lens with an NA of 0.6 can be enlarged to about 0.75 degrees. As a result, even when an error of about 0.25 degrees is caused in installing an optical head, a tilt up to 0.5 degrees, actually caused by a deformation on a disk, is still allowable.
First, a method for producing a CD will be briefly described for comparison. By using a stamper on which an audio signal is recorded, a substrate, a side of which is used for recording the audio signal, is produced so as to be 1.2 mm thick by an injection molding method. Then, a reflective film made of aluminum or the like is formed on the audio signal recording surface by a sputtering method. Next, a photopolymer resin is applied on the reflective film and then irradiated with ultraviolet (UV) rays, thereby forming a protective film. A CD is produced in this way.
If an optical disk with a single thin substrate is formed, then the disk is likely to be deformed by the weight thereof. In order to prevent such deformation, two substrates are bonded with each other. By attaching the two substrates, not only the mechanical strength of the disk can be increased but also the capacity of the disk is doubled, because the area usable for recording/reproducing the information thereon/therefrom can be doubled.
A conventional method for producing such a disk formed by the bonding of two thin substrates will be described. First, by using a stamper on which information signals such as video signals and audio signals are recorded, a first thin substrate, a side of which is used for recording the information signals thereon, is produced by an injection molding method.
Then, a reflective film made of aluminum or the like is formed on the information signal layer by a sputtering method. Next, a photopolymer resin is applied on the reflective film and then irradiated with UV rays, thereby forming a protective film. Furthermore, a second thin substrate is formed by using a second stamper on which different information signals are recorded, and then another reflective film and another protective film are formed in the same manner as the above.
Thereafter, a hot-melt adhesive is applied onto the protective film of each of the two substrates by using a roll coater; the two substrates are bonded with each other so that the hot-melt adhesives applied on the surface of each of the protective films are bonded with each other; and then pressure is applied to the assembly, thereby forming a bonded disk obtained by assembling the first and the second substrates. This bonding method using a hot-melt adhesive is the same as the method used for producing a laser disk.
The disk thus formed has a configuration in which the two substrates are integrated by opposing the protective films of the two substrates to each other and forming a hot-melt adhesive layer therebetween.
The configuration of such a conventional bonded disk will be described with reference to FIG. 8. As shown in FIG. 8, an information signal layer 102 is formed on one side of a first substrate 101. A reflective film 103 made of a metallic material mainly composed of aluminum or the like is formed on the information signal layer 102, and a protective film 104 is further formed thereon. On the other hand, an information signal layer 106 is formed on one side of a second substrate 105. In the same way, a reflective film 107 and a protective film 108 are formed on the information signal layer 106. A hot-melt adhesive layer 109 is further provided between the protective films 104 and 108 opposed to each other, thereby integrally bonding the first substrate 101 and the second substrate 105.
In order to realize an alternative optical information medium, a method in which two information signal layers for recording/reproducing information thereon/therefrom are formed by irradiating and focusing a laser beam from the same direction is disclosed in Japanese Laid-Open Patent Publication No. 3-209642 and U.S. Pat. No. 5,134,604. The reflectance of one of the information signal layers which is located closer to the laser beam incoming side is reduced, so that a sufficient amount of the laser beam can reach the other information signal layer distant from the laser beam incoming side and the information recorded on the respective information signal layers can be independently read out. Therefore, by using an optical information medium with the same size as that of a conventional medium, the area usable as the recording region can be doubled and the recording capacity can be considerably increased.
With respect to such a conventional dual-layer disk, the operational principle of the disk where the two information signal layers are exclusively used for reproducing the information therefrom will be described. FIG. 9 shows a case where the laser beam is focused on one of the two information signal layers which is distant from the laser beam incoming side. In FIG. 9, an information signal layer 152 is formed on one side of a transparent substrate 151 made of glass, resin, or the like. A semi-transparent thin film 153 is formed on the information signal layer 152 so as to partially reflect the laser beam. Another information signal layer 180 is formed over the semi-transparent thin film 153 via a transparent material 154. Since it is preferable for a reflective film 181 formed on the information signal layer 180 to reflect substantially all of the incoming laser beam, the reflective film 181 is made of a metallic material such as aluminum. A protective film 182 made of photopolymer resin or the like is further formed on the reflective film 181. The laser beam focused and incident on the information signal layer 180 is denoted by 171.
In the case where the laser beam is focused on the information signal layer 180, the information signals recorded on the information signal layer 180 can be reproduced, while the laser beam is partially reflected by the information signal layer 152 as denoted by 172 before the laser beam reaches the information signal layer 180. In such a case, if the thickness of the transparent material 154 is sufficiently large, then the spot size of the laser beam on the information signal layer 152 becomes sufficiently large, so that the signals recorded on the information signal layer 152 cannot be reproduced, and the signals reproduced from the information signal layer 180 are not adversely affected by the signals recorded on the information signal layer 152. In addition, if the semi-transparent thin film 153 on the information signal layer 152 is formed so as to have a uniform thickness, then a local phase change is not generated in the incoming laser beam, and the diffraction phenomenon undesirable for reproducing a signal can be suppressed at a negligible level. On the other hand, in the case where the laser beam is focused on the information signal layer 152 closer to the laser beam incoming side, the laser beam is transmitted through the semi-transparent thin film 153. However, since the spot size of the laser beam on the information signal layer 180 becomes sufficiently large, the information signals recorded on the information signal layer 180 cannot be reproduced, and the signals reproduced from the information signal layer 152 are not adversely affected by the signals recorded on the information signal layer 180.
FIGS. 10A to 10C show a method for producing such a dual-layer optical information medium. The transparent substrate 151 having the information signal layer 152 formed on one side thereof is formed by an injection molding method or the like in the same way as a CD substrate. As shown in FIG. 10A, the semi-transparent thin film 153 is formed on the information signal layer 152 by a sputtering method or a vacuum evaporation method using a target 161. The target 161 is a material for composing the semi-transparent thin film 153 and is made of a metal such as Au and Al or a dielectric such as ZnS. Then, as shown in FIG. 10B, a photopolymer resin 154 to become the transparent material 154 is inserted between a stamper 162 for forming the information signal layer 180 and the semi-transparent thin film 153 formed on the information signal layer 152; pressure is applied to the stamper 162 so as to obtain a predetermined thickness of the transparent material 154; and then UV rays 164 are irradiated through the semi-transparent thin film 153 and the transparent material 154.
Next, as shown in FIG. 10C, the reflective film 181 is formed on the information signal layer 180 obtained by removing the stamper 162 by a sputtering method or a vacuum evaporation method using a target 165. In the case where the information signal layer 180 is to be exclusively used for the reproduction, the target 165 is made of a metallic material such as aluminum. On the other hand, in the case where the information signal layer 180 is to be used for the recording and the reproduction, the target 165 is made of a phase-changeable material or a magneto-optical material. In performing this production method, the transparent substrate 151 and the semi-transparent thin film 153 are required to transmit the UV rays. Finally, the protective film 182 is formed on the reflective film 181 by using the photopolymer resin.
Since a conventional bonded disk is produced by a hot-melt method, the cost is considerably increased because an additional equipment such as a roll coater for applying a hot-melt adhesive and a presser is required in addition to the apparatus for producing a CD. On the other hand, since a new type of disk suitable for the use in a car is required to be developed from now on, such a disk is required to be resistant to the environment where the temperature reaches up to about 80.degree. C. and the humidity reaches up to about 85% for a long time. However, the hot-melt adhesive is softened at such a high temperature and high humidity, so that the disk is adversely deformed to exceed the allowable disk tilt.
According to the conventional method for producing the dual-layer optical information medium, when the information signal layer distant from the laser beam incoming side is formed, a photopolymer resin is inserted between the stamper on which the information is recorded and the semi-transparent thin film formed on the information signal layer closer to the laser beam incoming side, and the UV rays are irradiated through the information signal layer closer to the laser beam incoming side while applying pressure to the resin. Accordingly, a process step for curing the resin with the UV rays and a process step for removing the resin from the stamper are required to be performed for producing each disk. In addition, a certain amount of time is required for removing the cured resin from the stamper and dust is likely to be attached to the resin, so that productivity is decreased, the cost necessary for producing the optical information medium becomes high and the optical information medium has many defects.